Correct base image to use Python's Alpine image

[xee5ch]

As it stands, the current image is slim, which is Debian-based. This minor correction will use the -alpine flavor of official Python docker images so the apk commands work.

Closes #196.

[ghost]

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Legend

[izar]

Changed to -slim due to a Snyk alert. Must have not tested, my bad. Reverting. Thanks for the PR!